Solar Towers Not Avian Annihilators

Solar power towers have had a reputation as alleged avian vaporizers since preliminary reports of birds being burned in mid-air at California’s Ivanpah solar thermal plant. Their reputation was muddied even more during tests early this year at SolarReserve’s Crescent Dunes power tower, whose intense beams of reflected light recently began generating power in Nevada. California public radio station KCET reported that as many as 150 birds were killed during one six-hour test in January. It is obviously upsetting to imagine birds ignited in the name of renewable energy. But avian mortality is a downside common to many modern human creations—including buildings, highways, and powerlines. Full data on bird mortality at Ivanpah, macabre as it might be, shows the death rate to be small and likely of little ecological significance. “The data does support a low level of avian mortalities and hopefully, through adaptive management and deterrence, it will go even lower,” says Magdalena Rodriguez, a senior environmental scientist with the California Department of Fish and Wildlife.

It was, in his own words, a “back-of-the-napkin-level” estimate. Smallwood had extrapolated up from 183 bird carcasses found at Ivanpah in April and May of 2014 by scaling two months of data out to a year and by applying generic correction factors for sources of unobserved mortality—such as when searchers failed to spot some carcasses, or when scavengers carried others off.

Leroy Walston, an ecologist with Argonne National Laboratory and coauthor of an April 2015 assessment of avian mortality at utility-scale solar facilities, says Smallwood appropriately took a conservation-oriented approach, “in the face of missing information.” However, as that missing information has been replaced with site-specific data from systematic bird and bat surveys, a less alarming picture has emerged.

After adjusting for missing carcasses using experimentally-determined correction factors for the Ivanpah site, the biologists estimated total annual mortality at 3,504 birds—an eighth of Smallwood’s estimate. They projected that collisions with site structures such as mirrors and towers likely killed 774 birds over the year, while burns from solar flux killed an estimated 707. They estimated another 2,012 succumbed to unknown causes, including natural desert mortality unrelated to the solar project.

Based on the three-level scale of biological impact used by the site’s avian advisory committee, Ivanpah’s biologists categorized the annual impact as “Low” since they detected “minimal or no potential to negatively affect local, regional, or national populations within a particular species or group of species.” Bat mortality is negligible.

Walston at Argonne National Lab affirms their characterization. He says bird mortality at Ivanpah is “slightly higher” than for an average wind farm on a per-megawatt basis and “falls short” compared to the “millions of birds killed by building and vehicle collisions across the U.S.”

(Smallwood declined to comment, stating in an email to Spectrum that he hasn’t looked at the Ivanpah numbers since his 2014 testimony.)

Walston cautions against generalisations based on the Ivanpah data, given the as-yet-limited information available on avian mortality from all configurations of solar power. He says biologists can’t yet predict with any confidence whether the average solar power tower will harm more birds than solar farms using photovoltaic (PV) panels, or even whether either is more deadly than the rooftop PV systems spreading across cityscapes.

What is clear from the data is that Ivanpah is not the ecological nightmare once feared. And its avian footprint may already be shrinking further as operators gain experience, learning to avoid particularly deadly operating modes and keeping birds away from the power towers.

One area where there has been a significant advance is in understanding how to avoid killing birds when power towers are in standby mode, says Craig Turchi, a senior engineer at the U.S. National Renewable Energy Lab and a coauthor on Walston’s solar impacts report. Crescent Dunes’ January 2015 incident occurred when the plant’s ground-based mirrors or ‘heliostats’ were pointed skyward, focusing their beams in mid-air above the 195-meter-tall tower.

During normal operation, says Turchi, the heliostats’ beams converge on the receiver at the top of the tower. That creates a donut-shaped zone where the solar flux exceeds the 50-kilowatts-per-square-meter level that can harm birds. (Actually vaporizing birds would require over 1,000 kw/m2.) But it also lights up the receiver, says Turchi, providing some indication of the threat zone.

In contrast, deadly zones created by standby beams converging in open air are a relatively invisible threat – but one that’s is easily avoided by programming the heliostats to form a more diffuse standby pattern. “That’s a very correctable situation,” says Turchi.

Ivanpah, meanwhile, may already have greatly reduced mortality with deterrence measures it began testing last year at Unit 1 of the triple-tower plant. The wide-ranging deterrents include broadcasts of recorded distress calls for species observed on site, LED lighting (which draws fewer of the insects that can attract birds), and spiked surfaces to make perching near the tower undesirable.

The site’s biologists reported at the July 2015 avian advisory committee meeting that avian mortality at Unit 1 in Spring of 2015 was 70 percent lower than in 2014, and also 72 percent and 67 percent lower than concurrent mortality observed at Units 2 and 3, respectively. The advisory committee agreed with Ivanpah’s recommendation to begin implementing deterrence at Units 2 and 3.

Rodriguez calls the deterrence data “promising,” but is looking for more documentation than just a few seasons’ worth of comparisons: “For us to have confidence in their effectiveness—since the project is long running—we’d like to have a full year of monitoring of deterrence on each unit.”

Ivanpah just completed the second of two years of mandated avian monitoring, and the advisory committee must now determine whether it will continue. Given the strategic future value of power tower technology in a world that’s shifting towards 100 percent renewable energy, Rodriguez’ call for at least one more year of monitoring seems like a wise idea.

This post was created for Energywise, IEEE Spectrum’s blog on green power, cars and climate